Digital halftoning is a technique for displaying a picture on a two-dimensional medium, in which small dots and a limited number of colors are used. The picture appears to consist of many colors when viewed from a proper distance. For example, a picture consisting of black and white dots can appear to display various gray levels. Digital printers, which were initially pure black and white machines with a very coarse resolution, have evolved to accommodate colors, finer resolutions, and more recently, more than one bit of information per pixel (referred to as “multibit” or “multi-tone”).
For electrophotographic (EP or Laser) printers, documents are typically screened using binary periodic halftones at specific picture element (PEL) resolutions and halftone Line Per Inch (LPI) frequencies. The documents are printed as binary periodic dots. Multibit ink control is available in many ink jet printers. Such a printer can print a small, medium, large, or no dot. The halftone screening may allow for this when converting from the continuous or binary format of an original image
Halftoning algorithms can be divided into three categories 1) point operations, 2) neighborhood processes, and 3) iterative processes. In the first category “point operations”, each pixel in the original continuous-tone image is processed independently producing a corresponding PEL in the output image. An input continuous tone image may have 8, 12, or even 16 bits to provide a color value or depth for one or more different color planes for each pixel. Screening of this “point operation” type is the most common halftoning implementation found in printers. This is the least compute intense halftoning process. The processing involves comparing the digital count of each PEL in the original contone (continuous tone) input image with one or more thresholds from a matrix of threshold values. The result is the assignment of a native tone at each pixel in the output-device rasterized image based on the comparison. The native tones are the different drop sizes available e.g. none, small, medium and large.
In the second category “neighborhood processes”, processing is applied to a region of PELs around the PEL of interest where the result of the processing is quantized to select the appropriate native tone. The most common algorithm in this category is “Error Diffusion”.
The third category “iterative processes” creates halftone images iteratively by adjusting the drop locations and sizes to minimize a perceptual error metric formed from the difference between the filtered halftoned and filtered contone images. Iterative methods are the most computationally intensive of all digital halftoning methods, but they yield significantly better output quality than ordered dithering and error diffusion. The third method has the potential to produce results where the detail in the halftoned image approach the level of detail in the contone image, whereas point operation screening, the most common, is much more limited, due to the fixed screen dot positions.